# Customization point object (since C++20)

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### Explanation

The name `cpo` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__cpo_fn`.

All instances of `__cpo_fn` are equal. The effects of invoking different instances of type `__cpo_fn` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `cpo` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `cpo` above, `__cpo_fn` models

- std::invocable<__cpo_fn, Args...>,
- std::invocable<const __cpo_fn, Args...>,
- std::invocable<__cpo_fn&, Args...>, and
- std::invocable<const __cpo_fn&, Args...>.

Otherwise, no function call operator of `__cpo_fn` participates in overload resolution.

### See also

- RangeAdaptorObject (RAO)
- RangeAdaptorClosureObject (RACO)

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